LS3 Small-Block Build - Monster Mouse, Part 1

Big-bore crowd unite!

With the success of our Modern Mouse series, we decided it was time to recognize the big-bore crowd. Loyal readers of Super Chevy will remember the Modern Mouse series started with a stock 5.3L LM7 that we transformed into a 383 stroker. Along the way, we tested all manner of performance upgrades on the cathedral-port-headed small-block, ranging from simple intake and cam swaps, to strokers and forced induction.

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The one limitation of Modern Mouse was use of the 5.3L block. Though we were able to bore and stroke it to 383 inches, the block limited bore size to 3.905 inches. It was possible to step up to the larger 6.0L truck block (LQ4 or LQ9), but even those are limited to 4.030-inch bores. The current LS3 motors (including L92, LSA, LS9 etc.) come factory-equipped with a 4.065-inch bore, so we decided the big-bore guys deserved their own series. After all, a great many performance products were designed specifically for the big-bore LS3 motors. Hence our new series, entitled Monster Mouse.

When GM introduced the LS3/L92 family, the two major changes were an increase in bore size and rectangular-port heads. The change from the LS1-style cathedral-port heads to the new rectangular-ports netted impressive flow gains. How big were the flow gains offered by the new LS3/L92 heads? In a recent flow test against a set of cathedral-port 317 truck heads (that shares the intake port with the 243 LS6), the as-cast rectangular-port LS3 heads offered gains as high as 70 cfm. Just bolting these stock LS3 heads on in place of the cathedral-port heads was worth 40 hp. The flow numbers indicate that a set of stock rectangular-port LS3/L92 heads will support 650 hp. In fact, dyno testing a set of stock LS3 heads on a wild 468 stroker netted almost 700 hp.

The big thing to remember here is that just because the heads offer the potential, doesn't mean they will make 700 hp on your mild LS3. While an extra 40 hp from an LS3 head upgrade might sound enticing, know that they were designed for big-bore motors and will not bolt onto your small-bore 4.8L-5.7L and will only just fit on the 4.0-inch bore 6.0L truck and LS2 combinations. The head swap would naturally require the matching rec-port LS3 intake manifold and offset intake rocker arms.

Given the use of the rectangular-port heads, Monster Mouse required a suitable big-bore-block. Lucky for us, the LS experts at Texas Speed came through with not just an LS3 block, but a complete LS3-based stroker sporting no less than 418 ci. We were excited about testing the new Texas Speed stroker, but needed something to compare it to. As luck would have it, we had access to a stock LS3 crate motor secured through Summit Racing. With the stock engine in hand, we decided that the ideal first test for Monster Mouse would be to run the stock LS3 motor, then modify it with ported heads and a cam, then install the same components on the Texas Speed stroker. This would provide readers with plenty of information on both stock and stroker displacement LS3 applications. We have plenty of future testing in store for the stroker, but for now, let's get this party started.

First up was the stock motor. The LS3 crate piece is essentially a Corvette milled pulled from the assembly line. Displacing 376 ci and rated by the factory at 430 hp and 424 lb-ft of torque, our test method would unearth significantly more power from the impressive small-block. Tested on the engine dyno at Westech, the LS3 was equipped with an electric water pump, long-tube headers and run devoid of any accessories. Also on the missing components list were the induction system ahead of the throttle body, the entire exhaust system and any accessories. Rather than the conservative factory tune, we also optimized the air/fuel and timing using a FAST XFI/XIM management system.

These changes, plus running the motor at a slightly colder water temperature, increased the power output from the rated 430 hp to 493 at 5,700 rpm and 484 lb-ft of torque at 4,700 rpm. Equipped with the rectangular-port heads, torque production from the 6.2L LS3 exceeded 450 lb-ft from 3,900-5,700 rpm. This gave us a good starting point, but even the stock-displacement LS3 had more to offer.

Though blessed from the factory with impressive head flow, we knew there was more power to be had from this combination. First on the list of any modifications to an LS3 should be a camshaft. With such impressive head flow, the LS3 will respond very well to a more aggressive cam profile. Since our awaiting stroker came from Texas Speed, we hit them up for a cam as well. Taking into account the piston-to-valve clearance offered by the factory components, it supplied a healthy hydraulic roller profile that featured a 0.644/0.614 lift split, a 231/236 duration split and a 112-degree LSA.

Though the cam certainly offered plenty of power alone, we teamed it with a set of CNC-ported L92 heads from Summit Racing. The CNC heads further improved the flow rate of the already good stock LS3 casting. The only thing missing from the ported heads was a suitable spring package, as they were supplied with factory valve springs. Brian Tooley Racing came through with one of its valve spring upgrades. The dual-spring setup eliminated some of the problems associated with a single beehive spring and offered plenty of seat pressure (160 pounds) and open pressure (450 pounds) for our application. The kit also included the necessary titanium retainers, keepers and seals to facilitate the upgrade.

Replacing the heads and cam on the dyno was simple enough, but we did take the liberty of installing new Fel Pro MLS head gaskets and ARP head studs. The CNC-ported L92 heads utilized the factory pushrods, but hardened pushrods would be recommended with the increased valve spring pressure. The factory intake manifold was deemed more than adequate at this power level so it was reused, but dyno testing did require use of a FAST manual throttle body.

With Westech's Ernie Mean still in command of the tune, he quickly had the new combination dialed and styled. When the dust had settled, the modified LS3 produced 569 hp at 6,500 rpm and 522 lb-ft of torque at 5,200 rpm. Though the gains were most prominent higher in the rev range, torque production exceeded 450 lb-ft from 3,600-6,600 rpm. Where the stock motor struggled to produce 484 lb-ft, the new combination exceeded 500 lb-ft from 4,200-5,800 rpm. The ported heads and new cam profile improved power through the entire rev range, but we were anxious to see how it compared to the increased displacement of the Texas Speed stroker.

The final test involved the stroker short-block from Texas Speed. The 418ci displacement was the result of combining a 4.070-inch bore with a 4.0-inch stroker crank. Stuck in the middle was a set of 6.125-inch forged connecting rods and a heck of a lot of precision machining and balancing. The great thing about strokers is not so much the potential for increased peak power (though that is always welcome), but the real reason strokers bring smiles is torque. Increased displacement brings more power everywhere, from right off idle all the way to redline. Though we all brag about the big peak numbers, it is the extra grunt lower in the rev range that can be enjoyed on a daily basis.

To illustrate the gains offered by displacement, the Texas Speed stroker was equipped with the very same components used on the stock-displacement LS3. The TS custom cam and CNC-ported heads from Summit Racing were installed on the stroker. Running the same heads, cam and intake but with 42 more cubic inches produced 609 hp at 6,200 rpm and 570 lb-ft at 5,100 rpm. Compared to the modified LS3, the stroker was up by nearly 40 hp and 50 lb-ft of torque.

The best thing about the added displacement was that the extra 50 lb-ft was available through most of the rev range, from as low as 3,500 rpm. There is no better feeling than stomping on the gas to summon up an extra 50 lb-ft of torque, except maybe having an extra 100 lb-ft of torque, but we'll examine that kind of gain when we add forced induction to the mix. For now, the Texas Speed LS3 stroker was belting out over 600 hp in normally aspirated trim and we have a number of other combinations left to try.

Power Numbers: LS3 Stock vs. LS3
Modified vs Texas Speed Stroker

Stock

Mod

Stroker

RPM

HP

TQ

HP

TQ

HP

TQ

3,000

240

420

250

438

255

446

3,200

253

415

263

431

280

459

3,400

273

422

285

441

312

482

3,600

298

435

309

451

346

504

3,800

323

447

335

464

376

520

4,000

348

456

367

482

411

540

4,200

374

467

403

504

444

555

4,400

399

477

428

511

474

566

4,600

424

484

449

513

495

565

4,800

442

484

471

516

517

565

5,000

459

482

494

519

541

569

5,200

472

477

516

522

564

570

5,400

484

470

533

519

583

567

5,600

491

461

547

513

594

557

5,800

492

445

556

503

601

544

6,000

490

429

563

493

605

529

6,200

485

411

567

481

608

515

6,400

481

395

568

466

605

496

6,600

475

378

568

452

NA

NA

6,800

464

357

562

434

NA

NA

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16 After a break-in procedure, the stroker from Texas Speed belted out 608 hp at 6,200 rpm and 570 lb-ft at 5,100 rpm. True to form, the stroked LS3 improved the power output through the entire rev range, with gains of 50 lb-ft available below 3,500 rpm. Using the same cam, the stroker made peak power 300 rpm earlier than the stock displacement LS3.

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